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<title>CLiki: Issue EQUAL-CIRCULAR-LIST</title>
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<span class="hidden">CLiki - Issue EQUAL-CIRCULAR-LIST</span>
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<div id="content"><div id="content-area"><div id="article-title">Issue EQUAL-CIRCULAR-LIST</div><div id="article"><a href="Issue.html" class="category">Issue</a>: EQUAL-CIRCULAR-LIST<p>Forum: Editorial<p>References: <a href="https://www.cliki.net/site/HyperSpec/Body/fun_equal.html" class="hyperspec">EQUAL</a><p>Category: CLARIFICATION/CHANGE<p>Edit history: 2004-07-20, Version 1 by Bruno Haible<p>Status: For CLiki consideration<p><dl>
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<dt>Problem Description:<p></dt>
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<dd>Under CLHS rules, (<a href="https://www.cliki.net/site/HyperSpec/Body/fun_equal.html" class="hyperspec">EQUAL</a> proper-list circular-list) may fail to terminate, but in all known implementation it will terminate<p></dd>
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<dt>Proposal (EQUAL-CIRCULAR-LIST:ALLOW):<p></dt>
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<dd>Change "<a href="https://www.cliki.net/site/HyperSpec/Body/fun_equal.html" class="hyperspec">EQUAL</a> may fail to terminate if x or y is circular" to "if x <em>and</em> y are circular", so that (equal t '#1=(nil #1#)) is required to terminate.<p></dd>
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<dt>Test case:<p></dt>
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<dd>(equal '(a b c) '#1=(a b . #1#))<p></dd>
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<dt>Rationale:<p></dt>
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<dd>All reasonable implementations of <a href="https://www.cliki.net/site/HyperSpec/Body/fun_equal.html" class="hyperspec">EQUAL</a> behave this way.<p></dd>
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<dt>Current practice:<p></dt>
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<dd>
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<a href="SBCL.html" class="internal">SBCL</a> and <a href="CLISP.html" class="internal">CLISP</a> implement ALLOW.<p></dd>
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<dt>Cost to Implementors:<p></dt>
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<dd>None.<p></dd>
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<dt>Cost to Users:<p></dt>
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<dd>None.<p></dd>
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<dt>Cost of Non-Adoption:<p></dt>
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<dd>Users who assume a normal recursive implementation of EQUAL are trapped into pitfalls.<p></dd>
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<dt>Benefits:<p></dt>
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<dd>Legalize existing practice.<p></dd>
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<dt>Aesthetics:<p><dt>Discussion:
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<ul>
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<li>Bruno Haible says: I agree with proposal ALLOW. It is common practice to use (equal proper-list circular-list). The standard should allow it.
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